Traditionally steel strip was coated with zinc and was then referred to as galvanised steel. Zinc coatings have long been supplanted by coating of an aluminium-zinc alloy. Such alloy coatings retain the sacrificial protection afforded by zinc enhanced by the corrosion resistance of aluminium. A typical coating alloy may nominally comprise 45% zinc and 55% aluminium.
To effect the hot dip coating process the steel strip is drawn through a pool of the molten coating alloy within an open topped bath. To control the passage of the strip into and then out of the pool of alloy, referred to hereinafter as the bath metal in accordance with conventional terminology, the strip is caused to pass under a sink roll submerged in said bath metal.
Conventionally the sink roll and its submerged supporting structure have been made of a corrosion resistant alloy steel, for example a commercially available steel designated grade 316L stainless steel. Even so the working life of the submerged components is relatively short due to the corrosive effect of the bath metal and the build-up of intermetallic deposits resulting from chemical reaction between the components and the bath metal.
Prior art FIGS. 1 and 2 of the accompanying drawings illustrate the results arising from the use of 316L stainless steel.
FIG. 1 is a schematic diagram of the microstructure 50 of portion of a sink roll of 316L stainless steel 51. It shows a deposit of a mixture of bath metal 52 and intermetallic compound 53 on the surface of a normal alloy layer 54, which includes iron, chromium, nickel and aluminium, and which forms when the sink roll is immersed in the bath metal.
FIG. 1 also show the presence of σ-phase grain boundary precipitates 55. 316L stainless steel 51, and most other stainless steels, are susceptible to the formation of σ-phase precipitates over extended immersion times, which make the steel hard and brittle. Furthermore the σ-phase precipitates are rich in chromium and molybdenum so that their growth causes depletion of those elements in the grains surrounding the σ-phase precipitates. The presence of such micro-cracks together with the depletion of the overall chromium and molybdenum in the grains leads to high dissolution rates of the steel when exposed to the molten bath metal 52. Such dissolution manifests itself as pitting and other erosion of the submerged components.
Because the deleterious effect of depositing of the intermetallic compounds 53 on the quality of the finished product it is necessary to dress the roll from time to time to remove the deposit. This dressing is an expensive process, and it requires the coating operation to be interrupted for the removal and replacement of the sink roll.
Prior art FIG. 2 illustrates a severely pitted sink roll support arm fabricated from 316L stainless steel. Of course a sink roll, because it contacts the strip being coated and the coating quality depends on the smoothness of the roll, would have to be withdrawn from service long before it reached the state of the arm appearing in FIG. 2.
To overcome the deficiencies outlined above it has been proposed to subject the sink roll to a nitriding process. Nitriding is a conventional process affecting a thin surface layer of the component being nitrided and comprises holding the component for long periods in a furnace having an ammonia atmosphere.
When a sink roll that has been subjected to a nitriding process is immersed in the bath metal, the nitrides react with the aluminium in the bath metal, so that in addition to forming the alloy layer, a layer of aluminium nitride forms on its outer surface. This aluminium nitride layer is stable and acts as a protective, adherent surface layer on the component.
Prior art FIG. 3 is a view similar to FIG. 1 in respect of a nitrided 316L stainless steel sink roll. The figure shows all of the features of FIG. 1, but also shows a nitrided layer 56 with an aluminium nitride surface layer between the mixture of bath metal 52 and intermetallic compound 53 and the normal alloy layer 54 that forms when the sink roll is immersed in the bath metal. It will be noted that FIG. 3 also shows the presence of σ-phase precipitates 55 in the microstructure.
Nitriding is beneficial in that the stable aluminium nitride layer renders the intermetallic compounds less adherent to the roll. This facilitates their removal by scraping and prolongs the periods between dressings of the sink roll. The aluminium nitride layer also acts as a protecting layer and limits pitting or erosion of the component. The disadvantage of the nitriding process is the expense, the expert ability required to perform it, and the long wait required for obtaining the finished component.